It's easy once you feel it once... you never forget the feeling. When you are away from your person of interest, you feel listless, empty, wanting--and your stomach drops when you miss them. That's when I know, anyway. When i'm away from someone I LOVE, I feel like I need to get right back to them. But love isn't a one-sided thing, nor is it a one-faceted thing. There's a lot of things that go into love, and the feeling is only one of them.